Either there isn't any drainage system, or else there is a sump with drawoff located near the shell (see standard API-650 details). It's not intended or expected that you'd be able to drain every last drop out of the tank that way.

If you want a drain-dry tank, either slope down to the center, slope down to an off-center sump or slope uniformly across the tank. If you use a cone up bottom and it is a large tank, multiple sumps can help.

thank you guys for your reply!
Is there a way to determine the number of sumps needed?
Plus the civil engineer of the client designed the concrete foundation as a disk slab under the tank and not not a beam ring. It is it possible to incorporate our sumps systems in such a foundation?

Detailing is a hobby,

API-650 is silent on this question. No requirements per code. You could have none, you could have 12. It is an owner / operator preference. Most tanks have one sump, in larger tanks I often see two or more sumps.

Putting one or more sumps in a slab foundation is not unusual. It is harder to retrofit a sump in a cured and hardened slab but it's just more work. You may want to use a shallow sump if the slab is thin.

Cone up in the centre leads to a ring of liquid or water around the edge of the tank.

In theory you only nee done location to draw off liquid a the lowest point, but in practice larger tanks need more as it is not possible to get a tank laser line flat once it's been built and tested.

So the civil engineer has provided a single thickness slab under the entire tank??

He clearly has more money than most tank builders...

You can incorporate sumps in any foundation but just need to detail them beforehand.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Thank you all for your replies!
Very helpful!

Detailing is a hobby,